Auxiliary load-carrying apparatuses are known in the art and have gained some limited recognition as a means for solving the problem of meeting state and federal regulations which prescribe load per axle and loading per-axle per-unit-displacement between axles (the so-called "bridge laws").
While the principle of providing operator-controlled auxiliary load-carrying apparatuses is simple enough, and while the need for such devices is generally recognized, the obtainment of certain constructional and operational parameters has not been so readily obtainable. For example, load-carrying devices which selectively move from vertical raised position to lower ground-engaging position, can unduly interfere with other normal and expected functions of the vehicle, i.e., they impart instability to the vehicle, causing it to swerve, or interfere with the steering, turning, and other maneuvering functions of the vehicle. Quite obviously this is an intolerable situation. Also, in raised or transport position auxiliary load-carrying devices tend to raise the center of gravity of the vehicle, making it unstable and unwieldy. A substantial overhead load has the effect of raising the center of gravity of the vehicle as a whole, so that centrifugal force during turning develops the greater effect of tending to tip the vehicle over, or introduces objectionable sway. Overturning a vehicle, such as a selftransit concrete mixer truck, is not an uncommon occurrence. The vehicle itself is inherently unstable. This undesirable condition is then compounded with other unstable-creating effects arising during turning of the vehicle with a sloshing load of concrete within the mixer bowl. There is, therefore, a risk of either overturning the vehicle or requiring that the vehicle operate at such low speeds, and with such degree of caution, that it slows down the normal delivery functions of the concrete by the driver. This is not to say prior art auxiliary load-carrying devices are inoperative; within narrow confines, they do operate, and do relieve at least a portion of the axle loading; but the apparatuses impose an unacceptable design tradeoff in that the benefits to be gained by reduction of axle loading are offset by cumbersomeness of the machinery for effecting the result, introducing problems of instability to the vehicle for its normal steering and transport functions, and, furthermore, create unstable conditions of steering and maneuverability of the vehicle whether the load-carrying apparatus is in raised or lowered positions.
At lowered position, the instability described generally results from the relatively inflexible nature of the frame and wheel support provided by the auxiliary load-carrying apparatus. The apparatus has a different steering radius as compared with the vehicle, with the result that the vehicle and load-carrying apparatus oppose each other during turns, the equipment is unduly stressed, and the normal functions of steering and maneuvering the vehicle are impaired. These unsolved problems have confronted the art for many years.